Total knee-joint prosthesis

ABSTRACT

A prosthetic knee joint includes a metallic rocker member with a horizontal pin and a stem depending therefrom for insertion into the tibia, the pin being journaled in a two-part bearing of synthetic resin whose upper part is rigid with an upstanding shank receivable in the femoral shaft. The bearing has a rear cutout facilitating a swing of the rocker member about the pin axis over an arc of about 110*, the bottom surface of the lower bearing part and the rear surface of the upper bearing part being cylindrically curved about this axis in contact with a complementarily curved concave top surface of a cradle, rigid with the stem, whose flat underside comes to rest at the top of the tibia. The surface portions of the bearing body in contact with the rocker member, including the cylindrical bottom and rear surfaces of its lower and upper parts, are provided with a wearresistant metallic coating, e.g. of chrome-nickel steel.

i United States Patent Lagrange et a1.

[ TOTAL KNEE-JOINT PROSTHESIS Choisy, 75013 Paris, both of France [22} Filed: Sept. 23, 1974 [21] Appl. No.: 508,613

[30] Foreign Application Priority Data Oct. 2, 1973 France 73.31528 [52] US. Cl. 3/l.91l; 128/92 C [51] Int. Cl. A61F 1/24 [58] Field of Search 3/1, 1.9,1.91, 1.911,

[56] References Cited UNITED STATES PATENTS 3,688,316 9/1972 Lagrange et al. 3/1.911 3,694,821 10/1972 Moritz 3/191 3,760,427 9/1973 Schultz 3/1.91

FOREIGN PATENTS OR APPLICATIONS 939.226 2/1956 Germany 128/92 C 1 Nov. 11, 1975 Primary Examiner-Ronald L. Frinks Attorney, Agent, or Firm--Karl F. Ross; Herbert Dubno [5 7] ABSTRACT A prosthetic knee joint includes a metallic rocker member with a horizontal pin and a stem depending therefrom for insertion into the tibia, the pin being journaled in a two-part bearing of synthetic resin whose upper part is rigid with an upstanding shank receivable in the femoral shaft. The bearing has a rear cutout facilitating a swing of the rocker member about the pin axis over an arc of about 1 10, the bottom surface of the lower bearing part and the rear surface of the upper bearing part being cylindrically curved about this axis in contact with a complementarily curved concave top surface of a cradle, rigid with the stem, whose flat underside comes to rest at the top of the tibia. The surface portions of the bearing body in contact with the rocker member, including the cylindrical bottom and rear surfaces of its lower and upper parts, are provided with a wear-resistant metallic coating, eg. of chrome-nickel steel.

6 Claims, 2 Drawing Figures Cr-Ni STEEL US. Patent Nov. 11, 1975 3,918,101

TOTAL KNEE-JOINT PROSTHESIS FIELD OF THE INVENTION Our present invention relates to a total knee-joint prosthesis of the general type disclosed in our prior U.S. Pat. No. 3,688,316, designed to replace a knee joint destroyed by disease or injury.

BACKGROUNDOF THE INVENTION The knee-joint prosthesis described in our U.S. Pat. No. 3,688,316 comprises a metallic rocker member with a stem depending from a horizontalpin for insertion into the tibia, the pin being journaled in a two-part resinous bearing body whose upper part is adapted to be imbedded and cemented into the femoral condyles, this upper part being rigid with an upstanding shank receivable in the femoral shaft. A rear median cutout in the two bearing parts facilitates a swing of the rocker member about the pin axis through an obtuse angle, this pin axis coinciding with the swing axis of the natural knee joint. The two bearing parts adjoin each other along a parting surface which substantially coincides with a horizontal axial plane of the pin, this plane bisecting a pair of journal bores defined by the bearing parts for holding the ends of the pin on opposite sides of the stem. Surfaces of the bearing body contacted or contactable by the rocker member are advantageously coated with a wear-resistant metallic layer. The stem of the rocker member is integral with a disk, spaced from the lower bearing part, whose flat underside forms a transverse shoulder coming to rest at the top of the tibia.

Although such a prosthesis works well in parctice, we have found that prolonged use tends to generate stresses which weaken the rocker member and may even lead to a rupture of the stem just beneath the journal pin.

OBJECTS OF THE INVENTION The principal object of our present invention, therefore, is to provide an improved construction for such a prosthetic knee joint designed to relieve the aforementioned stresses and to insure an extended service life.

Another object is to provide means in a prosthesis of this character affording greater stability in all working positions, even after extensive use.

SUMMARY OF THE INVENTION Our improved total knee-joint prosthesis differs from that of U.S. Pat. No. 3,688,316 mainly by the fact that the bottom surface of the lower bearing part is convex and curved about the swing axis, i.e. the axis of the journal pin, and that the disk on the stem is replaced by a cross-bar shaped as a cradle with a concave top surface in mating engagement with that bottom surface throughout the swing range which may encompass an arc of about 110. The two interengaging surfaces are preferably cylindrical and substantially coextensive in the axial direction, for maximum support.

A wear-resistant layer, such as that with which the other contact surfaces of the bearing body are coated, is advantageously provided also on the cradled bottom surface of the lower bearing part and on a similarly curved rear surface of the upper bearing part forming an extension thereof. For additional support of the seat-forming cross-bar, the stem of the rocker member maybe provided with a pair of opposite vertical ribs coplanar with the pin 'axis.

BRIEF DESCRIPTION OF THE DRAWING These and other features of our invention will now be described in detail with reference to the accompanying drawing in which:

FIG. 1 isa partly sectional front-elevational view of a prosthetic knee joint embodying our invention; and

FIG. 2 is a partly sectional side-elevational view as seen in the direction of arrow II in FIG. 1.

SPECIFIC DESCRIPTION The prosthetic knee joint shown in the drawing is generally similar to that disclosed in our prior U.S. Pat. No. 3,688,316 and the same reference numerals have been used for identical or corresponding elements. Thus, a substantially T-shaped integral metallic rocker member comprises a transverse journal pin 11 supported by a shank 10, both of circular cross-section, on a fluted vertical stem 13 which is to be inserted into the tibia of the wearer and cemented in place. Stem 13 also carries, below pin 11, a seat-forming cross-bar 12 integral with that stem, the flat underside of this cross-bar constituting a transverse shoulder designed to come to rest at the top of the tibia.

A bearing member of synthetic resin, such as highdensity polyethylene, consists of an upper part 14 and a lower part 15 adjoining each other along a parting surface 16 in the horizontal diametrical plane of pin 11. The upper bearing part 14 is encased in a metallic yoke including a top plate 17 and two lateral flanges l8, l9 flush with and imbedded in the end faces of this element, each flange extending downwardly beyond the parting plane 16 so as to bracket also the lower bearing part 15. An upward extension of plate 17 forms an upstanding fluted shank 20 receivable in the femoral shaft in which it is to be held by cementing so that the upper bearing part 14 is firmlyseated between the femoral condyles; the axis A of pin 11 then is in line with the mean physiological knee axis.

The lower bearing part 15 is traversed by four threaded bolts .21, secured to plate 17, which are engaged by countersunk nuts 26 at their lower ends. This part is also formed with a median vertical cutout 22 aligned with a recess 23 in upper bearing part 15, the slot 22, 23 thus provided in the bearing body enabling a rearward swing of the rocker member 10-15 through an arc of about into a limiting position indicated in dot-dash lines in FIG. 2. I

The bottom surface 25 of lower bearing part 15 and the rear surface 28 of upper bearing part 14 have a convex cylindircal curvature of like radius centered on pin axis A. Cross-bar 12 has a concave top surface, centered with the same radius on this axis, which forms a seat for bearing body l4, 15. The two surfaces 25 and 28 together encompass an arc of almost and matingly engage the concavity of the cross-bar 12 over the entire swing range.

The wall surfaces of cutout 22 and recess 23 as well as the pin-engaging journal surfaces of bearing parts 14, 15 are coated with a wear-resistant and frictionreducing metallic layer 27, preferably of chrome-nickel steel, such a layer being also provided on the cylindrical supporting surfaces 25 and 28. The cradling of these convex surfaces on the concave seating surface of cross-bar 12 relieves the pin 1 1 and its journals of stress and effectively increases the available bearing surface by about 200 percent.

Stem 13 is formed with .a pair of lateral vertical ribs 29 which terminate at the underside of cross-bar l2 and lie in the plane of pin axis A, these ribs being of triangular profile as seen in FIG; 1; the upper edges of their triangle provide additional support for the crossbar 12 and through itffor bearing parts 14, 15 and for the extremities of pin 11 journaled therein. The stresses developing during use arethus more uniformly distributed over the entire prosthetic device.

We claim:

1. A total knee-joint prosthesis comprising:

' a metallic rocker member with a horizontal pin, a stem depending from said pin for insertion into-the I tibial shaft, and a cradle on said stem below said pin having a concave top surface centered on the pin axis, said cradle having a flat underside engageable with the upper end of the tibia;

. an upper bearing part adapted to be'imbedded in and cemented into the femoral condyles;

a lower bearing part adjoining said upper bearing part along a parting surface substantially coinciding with a horizontal axial plane of said pin, said 1 bearing partstogether defining a pair of journal bores bisected by said parting surface and centered on: said-pin axis for positively engaging the ends of said pin on opposite sides of said stem, said lower ing median cutout accommodating said stem for a g unidirectional swing of said rocking member from a depending vertical position, said bottom surface continuously engaging said top surface throughout the swing range for relieving said pin of stresses;

and i fastening means interconnecting said bearing parts for holding said pin in position.

2. A prosthesis as defined in claim 1 wherein said bottom and top surfaces are cylindrical and substantially axially coextensive 3. A prosthesisas defined in claim 1 wherein said outout is continued in said upper bearing part to provide an overall swing range of substantially 4. A prosthesis as defined in claim 1. wherein said bottom surface has a wear-resistant coating.

5. A prosthesis as defined in claim 1 wherein said stem has a pair of opposite vertical ribs coplanar with said pin axis supporting said cradle.

6. A prosthesis as defined in claim 1 wherein said. upper bearingpart has a rear surface adjacent said bottom surface curved about said pin axis with the same radius as saidbottom and top surfaces. 

1. A total knee-joint prosthesis comprising: a metallic rocker member with a horizontal pin, a stem depending from said pin for insertion into the tibial shaft, and a cradle on said stem below said pin having a concave top surface centered on the pin axis, said cradle having a flat underside engageable with the upper end of the tibia; an upper bearing part adapted to be imbedded in and cemented into the femoral condyles; a lower bearing part adjoining said upper bearing part along a parting surface substantially coinciding with a horizontal axial plane of said pin, said bearing parts together defining a pair of journal bores bisected by said parting surface and centered on said pin axis for positively engaging the ends of said pin on opposite sides of said stem, said lower bearing part having a convex bottom surface centered on said pin axis in mating engagement with the top surface of said cradle, at least said lower bearing part being provided with a rearwardly facing median cutout accommodating said stem for a unidirectional swing of said rocking member from a depending vertical position, said bottom surface continuously engaging said top surface throughout the swing range for relieving said pin of stresses; and fastening means interconnecting said bearing parts for holding said pin in position.
 2. A prosthesis as defined in claim 1 wherein said bottom and top surfaces are cylindrical and substantially axially coextensive.
 3. A prosthesis as defined in claim 1 wherein said cutout is continued in said upper bearing part to provide an overall swing range of substantially 110*.
 4. A prosthesis as defined in claim 1 wherein said bottom surface has a wear-resistant coating.
 5. A prosthesis as defined in claim 1 wherein said stem has a pair of opposite vertical ribs coplanar with said pin axis supporting said cradle.
 6. A prosthesis as defined in claim 1 wherein said upper bearing part has a rear surface adjacent said bottom surface curved about said pin axis with the same radius as said bottom and top surfaces. 